Graeme Robertson, Sulzer Downing and Mills: Generator repair and rewind

Sulzer Turbo Services

GasTurbine Conference , Brisbane, October 2013

Graeme Robertson – Head of Operations UK

Sulzer Dowding & Mills

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Sulzer Turbo ServicesLARGE HV MACHINE REPAIR OPTIONS

Repair to coil lead

Cutting coils out of circuit

Splicing in a coil side

Replace a failed coil [s] / bars

Complete rewind

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Sulzer Turbo ServicesFACTORS TO BE CONSIDERED

Machine Specific

Secondary damage, core

loss or impact

Impregnation system

Type of winding

Age of winding

General Factors

Physical size & access

Impact on other operations

Production loss and outage cost

/ Budget Availability

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Sulzer Turbo ServicesREPAIR TO COIL LEAD OR CONNECTIONS - 1

Simplest repair option if only one coil leg or connection is affected

Damage to adjacent coils increases the risk of repair escalating

Likely that secondary arc flash damage will be evident

Global VPI makes this process more difficult

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Sulzer Turbo Services2.5MVA, 2 MW, 8 Pole, 11KV

Global VPI Impregnation

Lead Arc Flash Damage

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Sulzer Turbo ServicesCUTTING COILS OUT OF CIRCUIT - 2

Will affect machine performance

Likelihood of reduced output

Excitation current will change

Out of circuit coils need to be isolated and insulated to prevent circulating

currents

Minimise air gap harmonics to minimise, noise, vibration, increased losses

In all cases Stator core requires to be fit for service as a result of failure

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Sulzer Turbo Services12 MW 4 Pole 11KV Compressor Motor

Stator Winding Earth Fault

Damage to sidewall insulation

Failed with multiple failed coils and earths

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Sulzer Turbo ServicesORIGINAL COIL CIRCUIT

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Coils reduced from 72 to 66

1 coil splice repair

STAR connected with 2 parallel circuits.

Coils / Phase parallel path 12 to 11

10% increase in gap density

Rating reduced from 12MW to 9MW

Minimal outage time available, required for production

CUTTING COILS OUT OF CIRCUIT

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Sulzer Turbo ServicesREDESIGNED COIL CIRCUIT

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Sulzer Turbo ServicesREDESIGN MODIFICATIONS TO CONNECTION RING

Connections of coils taken out of circuit

New Coil to Coil Connections@ Conn Ring

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Sulzer Turbo ServicesMODIFICATION TO NON CONNECTION END COIL

Cut Coil Knuckle@ Non Conn End

Cut Coil Isolated & Insulated

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Sulzer Turbo ServicesSPLICE IN COIL SIDE - 3

Straight forward with top coil side, can be achieved on bottoms legs

Must maintain strand continuity

Must maintain strand, turn & ground insulation levels

Must maintain coil bracing, coil to coil, coil to support arm

Must not restrict airflow – staggered scarfing

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Slot Cleaning is critical

Care taken to avoid damage to adjacent coils

SPLICE IN COIL SIDE

Lift Centre strip packing

Vacuum to remove all debris before, repacking and wedging

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Sulzer Turbo ServicesSTAGES OF SPLICING IN COIL

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No change in performance

Easier with half bars, diamond coil requires care / process

Must lift all coils in coil pitch [Diamond]

More coils lifted for WAVE winding

Depends upon condition of original winding, evolute / nose stress

Very difficult for global VPI windings, hard coil cells, prone to cracking

during removal

REPLACE A FAILED COIL / BARS - 4

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Sulzer Turbo Services120 MW 2 Pole 13.8KV Gen

Connections un-brazed to allow bar testing and subsequent removal

Stator bar being jacked out of slot

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Secondary Epoxy Injected

Replace Failed Coil

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Significant number of top bars [25] which had to be removed

Finished stator with bracing and blocking all restored, new connection caps fitted

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Sulzer Turbo ServicesREPLACE FAILED COIL

Stator bar being jacked out of slot, note the use of multi point jacking, the fit of coil and VPI system meant that the coil had to be drilled and tapped to facilitate removal without damage to core

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Performance maintained or Increased

If possible engineer out the previous failure mechanism – [ increase

interturn or sidewall insulation]

Restores Winding & Bracing system to new condition

Winding is a ‘consumable’ of a Motor or Generator, has a life span, which is

determined by loading / cycling / starts /environmental conditions /

maintenance procedures

New Windings will obviously extend the machine expected lifespan

FULL STATOR REWIND - 5

Sulzer Dowding & Mills 2013 <Copyright © Sulzer> | slide 23

Sulzer Dowding & MillsProduction Platform – Generation Problems


Sulzer Dowding & MillsOverview

Total generation capacity of 35MW

GT 4 was to be returned to Service after period of inactivity, this was a 15MW, 6.6kv, 4 pole Generator driven by an Avon Engine

Provided critical power for Sea Water Injection Pump motors

Platform required SWI Pump Motors running to support production

Module Roof was not watertight !

Generator Anti-Condensation Heaters were defective !


Sulzer Dowding & MillsBringing back to service…

Pre-commission checks identified very low IR and no PI.

1000v DC Test IR value 0.2MΩ & breaking down after 4 minutes

As our engineers were being mobilised to site, platform staff spun the turbine generator for a period of 4 hours unexcited to determine if dampness / condensation was the issue.

Retest did not show any difference in the IR or PI levels

Complete winding was wet with an earth fault present


Sulzer Dowding & MillsInvestigating the problem

Our Service Engineers then carried out a methodical check:

opened alternator for internal inspection of rotor / stator for any evidence of ingress of contamination

cables and all connections from the cubicle to the alternator terminal box, removed all clamps / post connections

disconnected the star point / split phase connections

split low reading phase coils to form 2 halves for inspection & test

continue to split until defective coil or coils were located

identify the defective area / coil section


Sulzer Dowding & MillsThe Defective Coil

The defective coil was identified by a process of elimination, and located at 4 o'clock position when viewed from DE, fault was specific to one diamond coil.

The stator winding had 66 coils connected in 2 parallel circuits

The winding had 5 & 6 coil groups

We proposed an in-situ repair potentially saving 16 weeks on time scale and circa £700k on rewind costs

A primary objective was to minimise outage time to get SWI pumps operating ASAP.


Sulzer Dowding & MillsThe Turbo Generator

Generator with end covers partially removedto expose Stator Winding


Sulzer Dowding & MillsSource of the Problem

Generator Stator Coils ready for splitting to locate defective coil


Sulzer Dowding & MillsWinding Modification

4 pole 3 Phase Generator

66 Slots & 66 Coils

2 Parallel Circuit Star

12 Coil Groups 5 coils & 6 Coils

2 Coil Groups in each parallel path 1 with 5 coils the other with 6 coils

Failed coil 2nd in 6 coil group


Sulzer Dowding & MillsWinding Modification

4 pole 3 Phase Generator

66 Slots & 60 Coils

2 Parallel Circuit Star

12 Coil Groups all 5 coils


Sulzer Dowding & MillsOur Solution..

We calculated what would be the effect of cutting out 6 coils, 2 per phase, on the performance of the Generator

It was necessary to take 2 coils from each phase, to prevent circulating currents and kept the winding balanced

We assessed that a maximum 10 % loss of output would result in changes

Results were relayed to Client who approved the process and methodology of repair.


Sulzer Dowding & MillsThe Completion..

Work was completed in 14 Days utilising 24 hr working :-

The generator was part dismantled, rotor remained in position An environmental habitat was erected around the stator Stator winding configuration was changed and reconnected,

the defective coil and 5 other coils removed from circuit. Stoving heaters were applied to dry the rest of winding The stator winding was cleaned, dried, and spray varnished The machine was fully built and prepared for test

Generator returned to service with slow increase of load from 2 MW to 7MW over a shift and then up to 13.8MW

Machine was handed back to client after successful commissioning test, with some alterations to protection relays and AVR settings.


Sulzer Dowding & MillsConclusion

Drilling Program was at risk due to lack of generation capacity

Failure to complete the ‘well work-over’ in time put the complete well at risk, including future oil revenues

Failure to provide power for the SWI Motors would result in loss of oil production

Generator Repair saved 16 Weeks of down time and circa £700k

Generator operating satisfactorily 15months later

Sister 15MW generator rewound this year

Scheduled to return 2014 to rewind this generator

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Sulzer Turbo ServicesBEST PRACTICES – SITE WORKING

LOCAL SITE CONDITIONS REQUIRE

Simple enclosure to create clean room

Controlled environment with some or all of:-

Air Conditioning De-humidifiers Heaters Work platforms Positive pressure Air Filtration Lighting Power outlets Tool storage

MITIGATING FACTORS

Remote Locations

Temperature & Humidity

Effects of high altitude

Hazardous Atmospheres

Noxious fumes

Political Situation (Risk)

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Sulzer Turbo ServicesBEST PRACTICES - STATOR WORKS

Stator core fit for 20-30 years

Fit of coil in slot

Resistance between coil & stator core

Tightness of wedging / size and position of ripple springs

Stress grading tape in correct position

Blocking & bracing of coil in endwinding

Continuity of sub-conductors

Insulation integrity / quality of coil / bar

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ConductingMiddle Strip

Bottom Coil Side

Wedge Top Packing

Top Coil Side

BEST PRACTICES – STATOR SLOT FILL

Conducting Vetronite

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Sulzer Turbo ServicesBEST PRACTICES - STATOR CORE

Test by power flux test or El-Cid method

Ensure tightness of lamination packs and cleanliness

Condition of air vents and security of duct spacers

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For tight fitting wedges max length 8”

Staggered under wedge packing

BEST PRACTICES – STATOR WEDGING

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Insulation Scarf Joint

Good scarfing of joints and staggering of scarfs

Avoid local build ups of insulation which would restrict air flow paths

BEST PRACTICES – STATOR SCARFING

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Maintain end winding blocking & bracing, maintain coil spacing

Ensure adequate clearances;

between coils

between connections

and to earth

Ensure adequate separation between phase coils

6.6kV 5.5mm 0.22”

11.0kV 8.0mm 0.32”

13.8kV 10.0mm 0.40”

BEST PRACTICES – STATOR BRACING / BLOCKING

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Winding Tests

DC resistance

Insulation resistance

Polarisation Index

Voltage withstand AC or DC

Tan Delta

Partial Discharge

BEST PRACTICES – STATOR TESTING

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Sulzer Turbo ServicesBEST PRACTICES – ROTOR REWINDING ON SITE

Two Options

New Rotor Copper

Utilising Original Copper

Removal of old insulation Validate forging Renew all slot, end-winding and under retaining ring insulation Reinsulate and rewind Maintain all symmetry of blocking & bracing Refit all fan blades and auxiliaries in original position Balance weights all accurately recorded and refitted Minor trim balancing may be required

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Sulzer Turbo ServicesBEST PRACTICES – ROTOR REWINDING ON SITE

When rewinding Rotor >100MW we would recommend high speed balancing which usually cannot be achieved on site.

Similarly when fitting new retaining rings, we would also recommend high speed balancing to ensure the correct balance condition at speed and also to ensure settlement of the rings

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Sulzer Turbo ServicesHIGH SPEED BALANCING FACILITY


Sulzer Dowding & MillsQuestions ?

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Graeme Robertson, Sulzer Downing and Mills: Generator repair and rewind